1. Field of the Invention
The present invention relates to a laser beam machining apparatus. More particularly, the present invention relates to a laser beam machining apparatus that is designed to machine a workpiece with curved surfaces and which is capable of adjusting the height of a laser beam emitting nozzle in such a manner that the distance between the tip of the nozzle and the workpiece as detected with a distance sensor such as a sensor for detecting electrostatic capacitance therebetween will be held constant.
2. Background of the Invention
For a laser beam machining apparatus of the type contemplated by the present invention, it is necessary that the laser beam emitting nozzle be controlled to always have the proper attitude with respect to the workpiece while the laser beam is focused at a constant point on the work. To meet these requirements, the conventional art employs a sensor that is installed at or near the tip of the nozzle for detecting the distance (the amount of gap) between the nozzle tip and the workpiece, with the nozzle being moved so as to keep the nozzle-to-work distance (or gap) constant, thereby permitting the laser beam to be properly focused on the work surface.
If the laser beam machining apparatus of interest is solely intended to machine flat plates, controlling the nozzle-to-work gap is very easy to accomplish because it usually involves moving the nozzle relative to the work surface only in the vertical direction. However, in an apparatus for three-dimensional machining, it involves complicated procedures to control the position and attitude of the nozzle since the control must be effected in a three-dimensional fashion. The complexity of the control system will be further increased if one wants to control the amount of the gap between nozzle and work surface on a real-time basis and to achieve rapid response to variations in the gap. Furthermore, attempts to satisfy these needs have often introduced instability in the operation of the servo system and resulted in failure to ensure high-speed and reliable machining.
Japanese Patent Application (OPI) No. 44487/1982 (the term "OPI" as used herein means an unexamined published Japanese patent application) proposes a capacitive sensor in the form of an annular electrode fixed around the tip of the laser-emitting nozzle, and Japanese Patent Application (OPI) No. 54487/1984 shows a capacitive sensor in the form of a separate electrode embedded in the tip of the nozzle. These sensors perform contactless detection of the capacitance (electrostatic capacity) between the electrode and the surface of the work W and by controlling the height of the nozzle with a servo mechanism so that the sensor will produce a constant output, the distance between the work W and the nozzle can be held constant. This method which enables contactless detection of the nozzle-to-work distance has proved very effective in the machining of a workpiece in flat plate form.
However, if the surface of work W has a complexly curved configuration with many high spots and recesses, the nozzle with the annular sensor is unable to gain access to very narrow valleys in the surface of the work W, with the result that some part of the work is left unmachined.
The nozzle having a separate sensor attached to the nozzle tip has such a geometry that it can have access to valleys in the surface of the work W as shown in FIG. 14. However, as the nozzle is brought close to the wall of the valley, the changes in the capacitance C.sub.x between the lateral side of the nozzle (i.e., the lateral side of electrode A) and the work W and in the capacitance C.sub.Y between the work W and a conductor wire a to the electrode A are added to the capacitance C.sub.L between the end face of electrode A and the work W which is to be measured for correct control of the nozzle-to-work distance. As a consequence, the capacitance detected by the sensor is greater than C.sub.L and the height of the nozzle is improperly controlled to a position that is offset from the desired height, with the result that laser beam machining cannot be accomplished at all or significant variations are introduced in the quality of the machined workpiece. Since the values of C.sub.X and C.sub.Y vary with the three-dimensional geometry of the work W, the correct amount of nozzle-to-work gap cannot be detected and the height of the nozzle relative to the work W cannot be held constant for the purpose of maintaining optimum machining conditions.